An Ultra Low Emittance DC/RF Photoelectron Gun

70868S02-II The radio frequency (RF) photoinjector is one of the great accelerator inventions in recent years. Unfortunately, its performance is limited by space charge effects, particularly at the cathode where electrons are first created at thermal energies. The lowest emittance achieved so far is on the order of a few mm-mrad. However, future linear colliders and 4th generation light sources require the emittance to be much lower. In this project, electrons created at a photocathode will be accelerated through a small (~mm), high-voltage (~250 kV), short-pulse (<1ns) dc gap to nearly relativistic energies with a very high accelerating gradient (~GV/m). Immediately after the rapid dc acceleration, the electron pulse will be injected into an integrated, emittance compensating, RF cavity for subsequent acceleration. An order of magnitude or more improvement in beam brightness is expected. Phase I used simulations to demonstrate that the proposed hybrid photoinjector can initially accelerate a low emittance electron beam to relativistic energies with a short pulse, high gradient, electrostatic dc gap, and that high-energy injection of the beam into several designs of a RF gun cavity indeed reduces the overall beam emittance by up to an order of magnitude. Phase II will perform experimental demonstrations of these effects, first with a 70 kV dc gap alone, and then in conjunction with a ballistic bunch compression RF cavity. After completion of the initial tests, a higher voltage (250 kV) transmission line and dc gun head will be designed, fabricated, and tested in a combined dc/RF gun. Commercial Applications and Other Benefits as described by awardee: An ultra-low emittance dc/RF gun would simplify the design and/or increase the luminosity of future linear colliders for high energy physics research. Also, the dc/RF gun should increase the brightness of 4th generation light sources that produce ultrafast x-rays for time resolved materials research, protein crystallography, and advanced medical and industrial applications.